36
edits
434: Difference between revisions
Jump to navigation
Jump to search
no edit summary
No edit summary |
Deanhuster (talk | contribs) No edit summary |
||
| Line 25: | Line 25: | ||
and Vgs of -0.26 V. The input amplifier has unity gain, high input impedance, and an output impedance of | and Vgs of -0.26 V. The input amplifier has unity gain, high input impedance, and an output impedance of | ||
about 120 Ω. The input amplifier drives a Tek-made differential amplifier chip, part number [[155-0050-01]]. | about 120 Ω. The input amplifier drives a Tek-made differential amplifier chip, part number [[155-0050-01]]. | ||
The original instrument design utilized an all-discrete switching power supply, making this an incredibly light portable scope. Unfortunately, this power supply was extremely difficult to troubleshoot when it went belly-up. At serial number B500000 and later, the design of the supply was simplified to use the same basic circuitry as found in the 7704A using a Tek-manufactured controller chip. | |||
The 434 was probably one of the worst scope designs to come out of Beaverton. As a bench tech, it was this model that caused me to thing that Beaverton should send all new mechanical engineers out into the field to work at a Service Center for a few months to get an idea of good and bad mechanical designs. First of all, the 434 design departed from the tried and true 4-40 steel PCB mounting screws. Instead, it uses 3-48 brass screws in a design that requires a screw-holding driver to get the fasteners in place. Needless to say, brass screws are not magnetic and getting them started in some of the more obscure spots on the vertical preamp board taxes a technician's moral speech patterns. | |||
The 434 uses only two plug-in attenuators (both X10) in each channel, and most V/DIV switching is gain switching of the preamp. The 434 often cannot meet vertical gain specifications and some very low-value 5% tolerance resistors must be replaced to correct this. These resistors are not located in an easy-to-access area of the board. | |||
If the V/DIV knob skirt lamps go out, the vertical preamp board must be removed to replace them. This is a process involving the removal of a lot of the aforementioned brass 3-48 screws, the removal of some connectors, the unsoldering of a lot of wires and the removal of knobs and shafts all over that area of the front panel. Remember, it's always better to leave a VAR V/DIV knob on the shaft and instead remove knob and shaft as a unit by loosening it from the coupler on the pot itself. | |||
Another common problem involves the mounting hardware for the back casting. In nearly every 434 that I received in the Service Center, I had to tighten all of those mounting screws. There's one located under the cover for the line cord and fuse. I've had that screw come completely out, and held captive inside that small space, it caused all sorts of hate and discontent with those open line connections. | |||
==Specifications== | ==Specifications== | ||